ETH Zurich The IAB Stack Evolution in a Middlebox Internet (SEMI) workshop in Zurich in January 2015 and the follow-up Substrate Protocol for User Datagrams (SPUD) BoF session at the IETF 92 meeting in Dallas in March 2015 identified the potential need for a UDP-based encapsulation to allow explicit cooperation with middleboxes while using encryption at the transport layer and above to protect user payload and metadata from inspection and interference. This document proposes a set of architectural considerations for such approaches.